1. Field of the Invention
The present invention relates to orthogonal frequency division multiplexing (OFDM) systems and, more particularly, to equalization within such systems.
2. Description of the Related Art
Over the past decade, orthogonal frequency division multiplexing (OFDM) has been exploited for wideband communication over mobile radio FM channels, asymmetric digital subscriber lines (ADSL), high-speed digital subscriber lines (HDSL), very high-speed digital subscriber lines (VHDSL), digital audio broadcasting (DAB), satellite digital audio radio services (SDARS), digital television terrestrial broadcasting (DTTB), digital video broadcasting (DVB), and HDTV terrestrial broadcasting.
OFDM is a block-oriented modulation scheme that maps N data symbols into N orthogonal carriers separated in frequency by 1/TS, where TS is the block (i.e., OFDM symbol) period. Multi-carrier transmission systems use OFDM modulation to send data bits in parallel over multiple, adjacent carriers (also known as subcarriers, tones, or bins). The fundamental operational details of OFDM are covered in U.S. Pat. No. 3,488,445, incorporated herein by reference in its entirety. An important advantage of multi-carrier transmission is that intersymbol interference due to signal dispersion (also known as delay spread) in the transmission channel can be reduced or even eliminated by inserting a guard interval between the transmission times of consecutive symbols. This minimizes the need for an equalizer, which is typically required in single-carrier systems. The guard interval allows energy from reflected or multipath copies of a symbol to die out before a subsequent symbol is received.
The basic principle of OFDM is to split a high rate data stream into a number of lower-rate streams that are transmitted simultaneously over a number of subcarriers within a defined frequency band. Since the symbol duration can increase for each of the lower-rate subcarriers, the effects of time dispersion caused by multipath delay spreads are decreased relative to single-carrier implementations. Further, because the information in the data stream is spread out over a large number of different frequencies (frequency diversity), the transmission is robust to narrowband interferers and fades. As a result, intersymbol interference is significantly decreased. During the guard interval typically added between symbols to further decrease intersymbol interference, it is common to cyclically repeat the OFDM symbol.
In practice, the most efficient way to generate the sum of a large number of subcarriers for transmission purposes is by using an inverse fast Fourier transform (IFFT) at the transmitter. At the receiver, an FFT can then be used to recover the subcarriers. All subcarriers differ by an integer number of cycles within the FFT integration time, which ensures the orthogonality between them. This orthogonality is maintained in the presence of multipath delay spread. Because of multipath, the receiver sees a summation of time-shifted replicas of each OFDM symbol. Ideally, as long as the delay spread is smaller than the guard time, there is theoretically neither intersymbol interference nor intercarrier interference within the FFT interval of an OFDM symbol. In practice, relative motion between the transmitter and receiver, non-ideal transmitter characteristics, noise and interferers, and an indefinite delay spread contribute to random phase and amplitude of each subcarrier at the receiver. To deal with weak subcarriers in deep fades, forward error correction (FEC) across the subcarriers is typically applied. The resulting “coded” OFDM (COFDM) system provides additional burst error robustness. However, this robustness comes at the cost of data throughput as a consequence of the redundancy overhead of the FEC. Additionally, to deal with amplitude and phase variations, it is common to provide some degree of equalization.
Even with the aforementioned safeguards, at any realistic power level, for transmitters and receivers that may be moving with respect to each other and with respect to potential interferers, some signal blockage or data loss is still likely to occur. To minimize the effect of this data loss, additional non-collocated transmitters are used (spatial diversity). In addition, the effect of burst errors that exceed the coverage of the FEC can be minimized by transmitting redundant copies of the signal at different times (temporal diversity). In both cases, data recovery is accomplished by consolidating the data from multiple unerrored segments of these temporally and spatially diverse copies into a single coherent data stream. One method to accomplish this is to assign a confidence weight to each symbol in each independent spatial and temporal stream at a receiver based on a calculated SNR for the symbol. A weighted average is then calculated via a maximal ratio combining (MRC) scheme resulting in an overall improvement in the receiver bit-error rate. This improvement is known as diversity gain.
Depending on the implementation, different bit modulation schemes may be employed within the COFDM scheme. For example, digital audio broadcast (DAB) employs differential quadrature phase shift keying (DQPSK) modulation, while DVB primarily uses quadrature amplitude modulation (QAM). Each of these modulation schemes provides unique advantages with respect to transmission channel characteristics.
In COFDM systems, time domain equalization (TEQ) is normally implemented to reduce the inter-symbol interference (ISI) for either QAM or DQPSK coding. For QAM coding, it is common to employ frequency-domain equalization (FEQ) techniques to combat both amplitude and phase errors caused by channel conditions. FEQ techniques jointly update coefficients to adapt for amplitude and phase simultaneously. However, currently, the techniques which involve jointly updating both amplitude and phase are difficult to implement for systems that use DQPSK coding.